The present invention relates to automatic lathes having a hollow spindle for receiving an elongated workpiece and in particular to means for hydraulically operating a chuck located within the mouth of the spindle for selectively clamping the workpiece.
It has been known to provide a hollow spindle with a collet or a chuck which is operable by an axially movable hollow shaft located within the spindle and which is selectively reciprocated by a hollow tubular hydraulic double action chuck motor. The hollow central passage of the operating shaft and of the chuck motor make it possible for the lathe to receive an elongated workpiece. In general the hollow chuck motor is located in a housing fixed on the supporting wall for the lathe spindle and comprises a hollow cylinder rotatable conjointly with the spindle and a hollow piston connected to the shaft. The cylinder and piston provide a double acting hydraulic mechanism capable of pushing and pulling on the shaft dependent upon the supply of hydraulic media, such as oil or the like to the respective chambers of the cylinder. To supply such hydraulic media, the cylinder of the chuck motor is provided with a hollow cylindrical extension axially arranged therewith in which two supply ducts are formed, communicating respectively with the chambers of the chuck motor. The supply ducts open on the periphery of the cylinder extension and are axially offset with respect to each other. Surrounding the cylinder extension are two annular fluid connecting members fixed by suitable means to the surrounding housing. Each of the two annular connecting members are in communication with a source of hydraulic fluid under pressure by suitable conduit means. The cylinder extension is journaled in a suitable roller bearing fixed within the housing to pass rotatably through the fluid connecting members. Each of the fluid connecting members is provided on its inner surface with a ring-shaped groove aligned in a transverse plane with the respective opening of one of the supply ducts. A slide valve is interposed in the conduits leading from the connecting members to the source of hydraulic fluid so that the flow of fluid can be regulated and controlled to one or the other of the connecting members and thereby to one or the other of the pressure chambers of the chuck motor.
In order to insure that the supply of the pressurized fluid medium is properly delivered to the correct chamber of the chuck motor, the hollow cylinder extension must be journaled within the fluid connecting members precisely. As a result a small bearing gap is created between the stationery annular fluid connecting members and the rotating cylinder extension. Due to the arrangement of the ring-shaped grooves a total of three such annular bearing gaps is formed. These gaps are sealed by the passage of hydraulic fluid medium axially outward between the stationery and rotating members. To maintain this precise arrangement relatively large diameter roller bearings are required. As a result of such construction severe limits are set to the spindle velocity so that the high rotational speeds required in modern automatic lathes cannot be obtained. Moreover, the pressurized hydraulic media flowing through the bearing gaps becomes overheated at high rotational speeds, so that the energy used to heat the hydraulic fluid tends to significantly reduce the working torque of the spindle. It would indeed be possible to provide better operating conditions, in this respect, by increasing the cross section of the bearing gaps, however, this would cause a flow of large volume of hydraulic fluid through the roller bearings which would also result in an inadmissible heating of the hydraulic fluid.
It is the object of the present invention to provide a lathe of the type described which is provided with means for hydraulically actuating the chuck in which the disadvantages and defects of the prior art constructions are avoided.
It is a further object of the present invention to provide apparatus of the type described which permits extremely high rotational speed of the working spindle while avoiding excessive heating and torque loss.
It is a further object of the present invention to provide apparatus of the type described which is simpler in design and more economical to construct and to use than the prior devices. It is still another object of the present invention to provide apparatus of the type described wherein loss of operating capacity is prevented even in the event of large leakage of hydraulic fluid, due to storage, occurs.
The foregoing objects, other objects, together with numerous advantages of the present invention are set forth and will be apparent from the following disclosure.